bootstrap: finish project bootstrap — scale test, MPFB character, build docs

- Static-mesh scale test passes: 1 m = 100.00 cm, origin preserved, axis
  mapping east→+X / north→−Y / up→+Z (named presets ue_metric_v1 both sides).
- MPFB 2.0.18 acceptance passes: generated human + standard 137-bone rig
  with weights + v2 skin baked to 1024² diffuse; imports as SkeletalMesh
  with Skeleton, Material, and Texture2D in /Game/Test/Character.
- MPFB headless requires XDG_CONFIG_HOME at the Flatpak config tree;
  documented in Art/blender/README.md, probe in Tools/addon_compat_probe.py.
- Docs/build.md completed: fresh-checkout build, packaging, headless checks.
- Docs/setup.md reconciled with reality; ROADMAP/bootstrap closed out.

Tools/scale_test_export.py

@@ -0,0 +1,81 @@
+# Scale-test scene builder and FBX exporter for the Blender -> Unreal pipeline.
+#
+# Builds three 1 m cubes in the project geographic frame (EPSG:26911, origin
+# Hollywood x Highland, X east / Y north / Z up):
+#   ScaleTest_A  centred at (0, 0, 0.5)      - origin + 1 m size reference
+#   ScaleTest_B  centred at (10, 0, 0.5)     - +10 m east probe
+#   ScaleTest_C  centred at (0, 10, 0.5)     - +10 m north probe
+# and exports them as one FBX with the named export preset "ue_metric_v1".
+#
+# Run headless (see Art/blender/README.md for the Flatpak invocation):
+#   blender -b --factory-startup --python Tools/scale_test_export.py -- \
+#     --out /tmp/gigline/scaletest.fbx --save-blend Art/blender/scaletest/scale-test-<date>.blend
+
+import argparse
+import os
+import sys
+
+import bpy
+
+
+def build_scene():
+    scene = bpy.context.scene
+    for obj in list(bpy.data.objects):
+        bpy.data.objects.remove(obj, do_unlink=True)
+
+    mat = bpy.data.materials.new("M_ScaleTest_Red")
+    mat.use_nodes = True
+    bsdf = mat.node_tree.nodes.get("Principled BSDF")
+    bsdf.inputs["Base Color"].default_value = (0.8, 0.05, 0.05, 1.0)
+
+    specs = {
+        "ScaleTest_A": (0.0, 0.0, 0.5),
+        "ScaleTest_B": (10.0, 0.0, 0.5),
+        "ScaleTest_C": (0.0, 10.0, 0.5),
+    }
+    for name, loc in specs.items():
+        bpy.ops.mesh.primitive_cube_add(size=1.0, location=loc)
+        obj = bpy.context.active_object
+        obj.name = name
+        obj.data.materials.append(mat)
+        print(f"[scale_test] created {name} at {loc}")
+
+
+def export_fbx(path):
+    os.makedirs(os.path.dirname(path), exist_ok=True)
+    # Named export preset "ue_metric_v1" - change only via a new preset name.
+    bpy.ops.export_scene.fbx(
+        filepath=path,
+        use_selection=False,
+        object_types={"MESH"},
+        apply_unit_scale=True,
+        apply_scale_options="FBX_SCALE_NONE",
+        axis_forward="-Y",
+        axis_up="Z",
+        use_space_transform=True,
+        use_mesh_modifiers=True,
+        mesh_smooth_type="OFF",
+        path_mode="AUTO",
+        bake_space_transform=False,
+    )
+    print(f"[scale_test] exported FBX preset ue_metric_v1 -> {path}")
+
+
+def main():
+    argv = sys.argv[sys.argv.index("--") + 1:] if "--" in sys.argv else []
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--out", default="/tmp/gigline/scaletest.fbx")
+    parser.add_argument("--save-blend", default=None)
+    args = parser.parse_args(argv)
+
+    build_scene()
+
+    if args.save_blend:
+        os.makedirs(os.path.dirname(args.save_blend), exist_ok=True)
+        bpy.ops.wm.save_as_mainfile(filepath=args.save_blend)
+        print(f"[scale_test] saved {args.save_blend}")
+
+    export_fbx(args.out)
+
+
+main()